Aromatic carboxylic acids, such as terephthalic acid, isophthalic acid, and napthlene dicarboxylic acid are useful chemical compounds and are raw materials in the production of polyesters. In the instance of terephthalic acid, a single manufacturing facility can produce greater than 100,000 metric tons per annum as feedstock for a polyethylene terephthalate (PET) facility.
Terephthalic acid (TPA) may be produced by the high pressure, exothermic oxidation of a suitable aromatic feedstock such as para-xylene. Typically, these oxidations are carried out in a liquid phase using air or alternate sources of molecular oxygen in the presence of a metal catalyst or promoter compound(s). Methods for oxidizing para-xylene and other aromatic compounds such as m-xylene and dimethylnaphthalene are well known in the art. These oxidation reactions will typically produce reaction gases generally comprising oxidation reaction products such as carbon monoxide, carbon dioxide, and methyl bromide. Additionally, if air is used as the oxygen source, the reaction gases may also comprise nitrogen and excess oxygen.
Some processes for the production of TPA also employ a low molecular weight carboxylic acid, such as acetic acid, as part of the reaction solvent. Additionally, some water may also be present in the oxidation solvent as well as being formed as an oxidation by-product.
Oxidations of this type are generally highly exothermic, and although there are many ways to control the temperature of these reactions, a common and convenient method is to remove the heat by allowing a portion of the solvent to vaporize during the reaction. The combination of the reaction gases and the vaporized solvent is sometimes referred to as oxidation reaction offgas. The oxidation reaction offgas contains a considerable amount of energy, and it is often desirable to efficiently recover energy contained in the offgas.
For example, JP 56-40636A describes an “oxidation reaction waste gas” as containing large amounts of moisture and vaporized aliphatic carboxylic acids, and also containing a small amount of bromine which all have corrosive properties. And although JP 56-40636A subsequently indicates that corrosion related problems can be avoided by constructing equipment from titanium or another corrosion resistant material, there are other examples of workers who have avoided corrosion issues by removing some or all of the components of the oxidation reaction offgas prior to introduction into a power recovery device.
In an article by Reumers, “Energy Conservation at Amoco Chemicals”, Journal A, Vol. 25 (3), 1984, an oxidation reactor offgas is passed to a condenser to remove the condensable components (i.e. low molecular weight carboxylic acid and water) and thus form an offgas comprising primarily nitrogen. The condenser offgas, which is composed almost exclusively of noncondensing gas or gases, is then passed to a turboexpander for energy recovery. In U.S. Pat. No. 5,723,656, an oxidation reactor offgas is passed to a high efficiency separation apparatus which removes most of the reaction solvent from the oxidizer offgas prior to directing to the resulting gaseous product which contains water vapor to a turboexpander.
Also in some processes for the manufacture of aromatic carboxylic acid, for example terephthalic acid, a desirable result is the efficient removal of the excess water generated by the exothermic liquid phase oxidation. It is usually desirable to maintain a consistently low water concentration, typically below 10 weight percent, in the oxidation reaction zone for the oxidation reaction to continue at a reasonable rate. The primary oxidation reaction produces one mole of water per mole of carboxyl moiety produced. In addition, there are other side reactions which release water, e.g. the direct oxidation of the solvent to form by-products, and water may be added to the process for other reasons such as scrubbing offgas for solvent recovery. Typically, at least a portion of the oxidation offgas either as vapor or condensate is usually directed to a fractionation device, typically a distillation column, to separate water from the primary solvent (e.g. acetic acid) so that the water concentration in the oxidation reactor is not allowed to build up.